1. Field of the Invention
The present invention generally relates to a front wheel differential lock control system for a straddle-ride type four-wheeled vehicle and, more particularly, toward such a control system in which a switch between lock and unlock states of a front wheel differential gear is accomplished by the drive force of an actuator.
2. Description of Related Art
A straddle-ride type four-wheeled vehicle as an ATV (All Terrain Vehicle) may lose drive force during traveling through mud, a bog, quagmire or the like, for example, in four-wheel drive (4WD) state because of spinning of one of front wheels. In such a case, left and right axles have heretofore directly been connected to each other by locking (diff-locking) a differential gear of the front wheels, thereby ensuring the drive force. The switching between the unlock and lock states of the differential gear is performed generally by allowing an actuator (a diff-lock actuator) such as a motor or the like to drive a differential locking mechanism in response to the operation of an electric switch installed on a steering handlebar or the like.
Incidentally, if the differential gear of the front wheels is locked, a rotational difference does not occur between the left and right wheels. This leads to a state unsuitable for turning traveling and high-speed traveling. Therefore, it is conceivable that switching to the lock state of the differential locking mechanism is permitted only under certain predetermined conditions.
Japanese Patent Laid-open No. 2002-172951 discloses a differential lock control system that permits switching the differential locking mechanism into the lock state, that is, that permits the drive of a diff-lock actuator only if vehicle speed detected by a vehicle speed sensor is equal to or less than a predetermined value.
However, the technology described in Japanese Patent Laid-open No. 2002-172951 detects vehicle speed on the basis of the rotational speed of a wheel detected by a rotation sensor installed on the wheel. In the state where the wheel is slipping, the vehicle speed detected by the vehicle speed sensor may be high although the actual speed of the vehicle body is low. Thus, a phenomenon may probably occur in which switching to a differential lock state is not permitted in the slip state where an occupant desires the switching into the differential lock state.
The differential locking mechanism is usually configured such that even if a diff-lock actuator is actuated, switching into a lock state will not be completed as long as a difference in rotation number occurs between left and right wheels. Although differential lock control is exercised during low vehicle-speed, vehicle speed may probably increase with a lock waiting state remaining. To prevent switching into the lock state in the high vehicle-speed state, this case needs the following. A threshold level of vehicle speed at which the drive of the diff-lock actuator is permitted is allowed to have hysteresis. Alternatively, engine control is exercised so that vehicle speed does not rise over a predetermined value. Thus, there is a problem of complicated control contents.